The present invention relates in general to fuel burners, and in particular, to a new and useful burner for the combustion of coal, oil or gas, which simultaneously achieves low NO.sub.x emissions with a relatively short flame.
Low NO.sub.x coal-fired burners rely on principles of air staging and/or fuel staging to reduce formation of nitric oxides during combustion. In either case, it becomes necessary to permit a portion of the combustion process to take place in fuel-rich/oxygen-deficient conditions such that reactions can take place to form N.sub.2 rather than NO or NO.sub.2. A good example is the burner disclosed in U.S. Pat. No. 4,836,772 which achieves very low NO.sub.x emissions by use of air staging and fuel staging.
Air staging is achieved by a dual air zone burner barrel arrangement which enables regulation of air introduction to the fuel. Consequently, not all the air introduced through the burner is permitted to mix immediately with the fuel, but rather its introduction is controlled to take place gradually.
Fuel staging is achieved by introduction of the fuel in a controlled fuel rich zone, which results in partial combustion and generation of hydrocarbon radicals. These radicals proceed to mix with the products of combustion and reduce NO.sub.x formed earlier in the flame. The combined effects are achieved by introducing the fuel jet axially into the combustion chamber, with sufficient momentum as to delay the mixing between fuel and air. An undesirable attribute of such a burner/process is the relatively long flame which results. Delayed air/fuel mixing tends to cause flames to become much longer than rapid-mixed high NO.sub.x flames. Elongated flames may them impinge on furnace walls leading to slag deposition, corrosion, and higher levels of unburned combustibles (flame chilling). These effects can have significant impacts on the operation, service life, and efficiency of combustion, respectively. Fuel staging is disclosed in U.S. Pat. No. 4,206,712.
To reduce flame length in low NO.sub.x burners, impellers can be installed at the exit of the coal nozzle. These serve to deflect the fuel jet, reducing axial fuel momentum and reducing flame length. However, NO.sub.x increases significantly. Another known burner disclosed in U.S. Pat. No. 4,400,151 separates the fuel jet into several streams which are accelerated and deflected at the nozzle exit. NO.sub.x performance is again improved, like the burner of U.S. Pat. No. 4,836,772 which uses an impeller. In addition, the burner in U.S. Pat. No. 4,400,151 provides for some fuel jet velocity control with questionable effectiveness. This design suffers from poor mechanical reliability.
Tests have shown the burner of U.S. Pat. No. 4,836,772 can produce a short flame with very low NO.sub.x, however, very high secondary air swirl is required to counteract the fuel jet momentum. The high secondary air swirl requires prohibitively high burner pressure drop.
U.S. Pat. No. 4,768,948 discloses an annular nozzle burner which produces a compact flame parallel to the burner axis. U.S. Pat. No. 4,428,727 discloses a burner for solid fuels having an axially moveable element which can vary the size of an annular outlet gap from the nozzle. An axially adjustable impeller is disclosed in U.S. Pat. No. 3,049,085.